Various kinds of automatic focus control devices of the so-called "active type" have been proposed and some of these have been put to practical use. Among these, active distance measuring devices project infrared rays. FIG. 1 schematically illustrates a typical example of a conventionally known measuring device which projects such infrared rays. Here, an infrared ray emitting diode 2 (hereinafter referred to as IRED for short) projects infrared light toward an object through a light projecting optical system 4. A light sensitive element 8 receives light reflected from the object through a light receiving optical system 10 and an infrared transmitting filter 12.
With the conventional device arranged in this manner, the IRED 2 is moved by some suitable known means, not shown, in the direction of arrow A from position a to another position b. The infrared light is projected along an optical path c when the IRED 2 is in position a and along another optical path d when the IRED is in the other position b. Therefore, the intensity of the infrared rays to be sensed by the light sensitive element 8 becomes the highest when the light is projected along an optical path e which encounters to the object 6 during the movement of the IRED 2. Assuming that the distance to the object 6 is to be measured on the basis of the principle of trigonometric distance measurement, the angle of projection, i.e. the moving position of IRED 2 at which the light sensitive element 8 most strongly senses the intensity of the infrared rays corresponds to the distance to the object. Since this operation is well known, description of further details of the operating principle of the conventional device are omitted herein.
The above description merely covers the principles of these conventional devices. However, in the actual application, the effects of external light cannot be ignored in spite of the use of the infrared transmitting filter 12. To solve this problem, in projecting the infrared light from the IRED 2, the infrared light is modulated and, in the meantime, the light sensitive element 8 is arranged to detect the reflected infrared light coming from the object by a synchronous detection process.
FIG. 2 shows an example of a circuit arrangement for this synchronous detection. This circuit includes a light receiving amplifier 14 which is arranged to produce an output by converting a photo current flowing through the light sensitive element 8 into a voltage through a feedback resistor 16; a capacitor 18 which eliminates the DC component of the output of the light receiving amplifier 14; a buffer amplifier 20 which determines the level of the AC component after elimination of the DC component; a drive circuit 32 which modulates the IRED 2 by causing it to flicker at a frequency of about 10 KHz; a sample and hold circuit 22 which is arranged to sample and hold the output of the buffer amplifier 20 when the IRED 2 is lit; another sample and hold circuit 24 which is arranged to sample and hold the output of the buffer amplifier 20 when the IRED 2 is extinguished and to produce an output by inverting it; an operational amplifier 26 which adds up the outputs of the sample and hold circuits 22 and 24; a capacitor 28 of a low-pass filter which eliminates the high frequency component of the output of the operational amplifier 26; and a buffer amplifier 30 for producing an output.
The operation of the circuit arrangement of the conventional device shown in FIG. 2 is as shown in the timing chart of FIGS. 3(1) to 3(9). FIG. 3(1) shows the on-and-off timing of the IRED 2. FIG. 3(2) shows the output signal of the light receiving amplifier 14. FIG. 3(3) shows the output signal of the buffer amplifier 20. FIG. 3(4) shows the sampling pulses to the sample and hold circuit 22. FIG. 3(5) shows the sampling pulses to the sample and hold circuit 24. FIG. 3(6) shows the sample and hold output signal of the sample and hold circuit 22. FIG. 3(7) shows the sample and hold output signal of the sample and hold circuit 24. FIG. 3(8) shows the output signal of the operational amplifier 26. FIG. 3(9) shows the output signal of the buffer amplifier 30.
When the reflected infrared rays strike the light sensitive element 8 according to the on-and-off operation of the IRED 2, a photo current in which an exterior light component and the reflected infrared light overlap each other flows through the light sensitive element 8. The photo current is voltage converted through the resistor 16 of the light receiving amplifier 14 into a voltage signal as shown in FIG. 3(2). The voltage signal then has its DC component eliminated through the capacitor 18 and the buffer amplifier 20 and is taken out as an AC signal as shown in FIG. 3(3). Meanwhile, the sample and hold circuits 22 and 24 to which this AC signal is to be supplied respectively receive pulse signals as shown in FIGS. 3(4) and 3(5). These signals are supplied to the circuits 22 and 24 according to the lighting and extinction timing of the IRED 2 as sampling pulses respectively. Accordingly, as a result of their sample and hold actions, the circuits 22 and 24 respectively produce sample and hold signals as shown in FIGS. 3(6) and 3(7).
The outputs of the sample and hold circuits 22 and 24 are added up at the operational amplifier 26. The output of the operational amplifier 26 is supplied to a low-pass filter as the wave form shown in FIG. 3(8). The low-pass filter, which consists of the capacitor 28 and the buffer amplifier 30, removes a ripple component from the input to give a wave form as shown in FIG. 3(9).
Therefore, the output of the buffer amplifier 30, while the IRED 2 is being moved in the direction of arrow A, constitutes a signal which reaches its peak when a light beam projected by the IRED 2 just impinges upon the object 6 whose distance is being measured. Then, it is possible either to measure distance or to allow a camera or the like to perform an automatic focus control action by correlating the peak position thus obtained with the moving position of the IRED 2.
However, the actual output of the low-pass filter consisting of the capacitor 28 and the buffer amplifier 30 is not always of a wave form completely devoid of a ripple component as shown in FIG. 3(9). Therefore, an erroneous peak might be detected before a correct peak position is found. To avoid incorrect action due to such erroneous peak detection, it has been practice to provide a dead zone for peak detection. In carrying out peak detection, a peak detection circuit according to the conventional practice first detects the peak of a wave form, such as the wave form shown in FIG. 3(9), for example, and then sees whether or not there appears an output that exceeds the detected peak within the predetermined period of time. If an output exceeding the first detected peak within the predetermined period of time appears, the first detected peak is determined as an error and the next peak is detected. Conversely, if an output that exceeds the peak after the lapse of the predetermined period does not appear, the peak detected first is determined as a correct peak.
The provision of a dead zone thus enables prevention of erroneous action due to erroneous peak detection. However, the provision of the dead zone results in a time lag. The time lag tends to cause an error in distance measurement or, in the case of an automatic focus control device, prolongs the length of time required for automatic focus control. In the case of a camera or the like, a photo taking lens is controlled after the lapse of a certain length of time after the actual occurrence of a peak. In view of this, the moving speed of the lens must be precisely controlled. This has necessitated the use of a governor or the like. Such an arrangement produces various inconveniences, such as a large noise produced when the photo taking lens moves, a large torque required for charging the automatic focus control system during winding of film and charging the camera, etc. Where a winding and charge-up action is accomplished by means of a motor or the like, the use of such an additional member imposes a large load. Further, in accordance with the prior art arrangement, the photo taking lens is moved concurrently with the movement of the IRED. This also has necessitated a complex structural arrangement for locking and unlocking a focusing system.